A gas treatment device for purifying exhaust gas is provided to a vehicle (e.g., automobile) in order to remove a toxic substance (e.g., carbon monoxide, hydrocarbon, and nitrogen oxide) included in exhaust gas discharged from the engine. Such a gas treatment device normally includes a processing structure (e.g., catalyst carrier) that is formed to have a tubular shape, a metal casing that accommodates the processing structure, and a holding material that is arranged in the processing structure, and placed in the gap between the processing structure and the casing.
The holding material that is placed in the gap between the processing structure and the casing is required to have a function of reliably (safely) holding the processing structure so as to prevent a situation in which the processing structure collides with the casing and breaks due to vibrations and the like that occur when the automobile is running, and a sealing function that ensures that leakage of unpurified exhaust gas from the gap between the processing structure and the casing does not occur. In order to reliably (safely) hold the processing structure, it is necessary to hold the processing structure with a specific surface pressure. However, a decrease in surface pressure occurs due to repeated compression caused by heating.
A holding material produced by forming ceramic fibers (e.g., alumina fibers or silica fibers) in the shape of a mat having a specific thickness has been mainly used as the holding material. A method that utilizes two types of mats in combination has been proposed in order to provide an additional function. For example, Patent Literature 1 discloses using mats that differ in length in combination in order to improve a winding capability. Patent Literature 2 discloses using mats that differ in thermal resistance in combination in order to improve thermal resistance. Patent Literature 3 discloses using a mat that includes bio-soluble fibers and a mat that includes non-respirable fibers in combination in order to improve safety.